Dengue was commonplace in Florida until the 1930s. Air conditioning, window screens and better mosquito control helped break the dengue cycle.

But in 2009, things changed. A dengue outbreak in Key West sickened at least 28 people. Investigators found that about 5 percent of 240 people they tested around Key West showed signs of having been infected by the virus that causes dengue.

Dengue is marked by fever, headache and pain in muscles, joints and bones. The illness can be excruciating. But infections can also be mild. Those can go overlooked or be confused with the flu. A blood test can conclusively diagnose dengue.

Since the Key West outbreak, doctors and public officials in the state have been on guard.

"We have better surveillance," says Carina Blackmore, interim state epidemiologist at the Florida Department of Health. "We believe we're picking up dengue that was occurring."